AprilTags not detected when close to camera

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Maybe you can get your anwser by reading through the paper, where the implementation is explained. Maybe one explanation is, that during the quad detection process only close enough lines are considered.

Robustness to occlusions and segmentation errors is handled by adjusting the “close enough” threshold: by making the threshold large, significant gaps around the edges can be handled. Our threshold for “close enough” is twice the length of the line plus five additional pixels. This is a large threshold which leads to a low false negative rate, but also results in a high false positive rate.

That would explain why reducing the resolution helps detecting the closer tag.